Recently, the use of terminals such as smartphones and tablet personal computers is being widespread rapidly. Those terminals are required to display much information of high image quality while keeping the screen size handy to carry about. To solve that, lateral electric field type liquid crystal display devices in the IPS (In-Plane Switching) mode and FFS (Fringe Field Switching) mode, which can provide a high resolution and an excellent viewing angle, are employed for their display devices, wherein an electric field almost parallel with a substrate of each display device is used so as to rotate liquid crystal molecules in a plane almost parallel with the substrate.
Generally, display devices in the IPS mode, which use a lateral electric field generated between a strip-shaped pixel electrode and a common electrode, are more excellent in viewing angle characteristics than display devices in the FFS mode, which use a fringe electric field generated by arranging a strip-shaped electrode on a tabular electrode. However, the IPS mode display devices can cause the following problems in pixels for achieving extremely high definition.
FIGS. 16 and 17 are a plan view illustrating a conventional IPS mode display device and its sectional view taken along the line XVII-XVII of FIG. 16, respectively (as a first example of conventional arts). In order to shield an electric field emitted by data lines 5, common electrode 1 is arranged so as to cover the data lines 5 with putting an insulating film between them. Pixel electrode 2 in a strip shape is arranged separately from the common electrode at a certain distance. FIGS. 16 and 17 further illustrate source electrode 3, thin-film semiconductor layer 4, scan lines 6, alignment direction of liquid crystal 7, lateral electric field 8, light-incident-side polarization plate 10, first transparent insulating substrate 11, gate insulating film 12, passivation film 13 and oriented film 14. In an excellent-definition liquid crystal display whose pixel size is about 90 μm or less, each of sub-pixels provided by dividing each pixel into R, G and B elements (R: sub-pixel for red, G: sub-pixel for green, B: sub-pixel for blue) becomes about 30 μm or less in horizontal size. Therefore, providing only one pixel electrode extending along the data lines, can increase the aperture ratio of pixels.
In many of conventional and general liquid crystal display devices in the IPS mode, a pixel electrode is arranged between common electrode parts arranged at the both side of a sub-pixel so as to make the spacing between the pixel electrode and each of the common electrode parts almost uniform. FIG. 18 shows electrode spacing between the common electrode and the pixel electrode for gradually decreasing sizes of one pixel under the condition that each data line 5 is 3 μm in width, common electrode 1 is 9 μm in width and pixel electrode 2 is 3 μm in width. Under this condition, the electrode spacing comes to 3 μm or less in pixels whose size is 50 μm or less. Such the pixels hardly secure the enough electrode spacing and can cause a significant deterioration of light-utilization efficiency, which is a problem.
On the other hand, JP-A No. 2006-267317 discloses a technique to enlarge the electrode spacing (as a second example of the conventional arts), wherein a set of a pixel electrode and a common electrode part is formed at each of the opposing sides of a pixel so as to form just one space between the pixel electrode and the common electrode in one pixel. FIGS. 19A and 19B illustrate such the conventional technique. FIG. 19A illustrates a plan view of one pixel and FIG. 19B illustrates its sectional view taken along the line XIXB-XIXB in FIG. 19A. This conventional technique provides the following structure shown in FIGS. 19A and 19B. In the structure, protrusion 101 is formed to cover each of data lines 106. Pixel electrode 105 and common electrode part 104 are formed on both side surfaces of each protrusion 101, respectively, so as to apply a lateral electric field to a space between the electrodes to drive liquid crystal. Further, FIGS. 19A and 19B illustrate glass substrate 100, first insulating film 102, second insulating film 103, second pixel electrode 105, first pixel electrode 107, storage capacity electrode 108, TFT 109, common electrode lines 110, gate line 111, first contact hole 112 and second contact hole 113.
Since there are no electrodes which cover spaces right above the data lines 106, this technique requires to make common electrode part 104 and pixel electrode 105 wider in order to avoid a leak of the electric field coming from the data lines 106. This structure drives liquid crystal by using a lateral electric field generated between sets of a common electrode part and a pixel electrode, but can cause a problem that an effective aperture ratio is hardly secured because of the above reason. Further, because this structure includes protrusions, more than the half of each pixel is occupied by the protrusion under the condition that the size of each pixel is 90 μm or less, which causes another problem that a treatment, such as a rubbing treatment, to define the initial alignment of liquid crystal molecules in an aperture section where no protrusions are arranged is hardly performed and it results in a deterioration of the image quality.
The present invention seeks to solve those problems.